Digital Subscriber Line (DSL) refers to data communication technologies that leverage the longstanding infrastructure of bundled twisted-pair copper wires that were originally deployed for analog telephone service as a way of delivering high-speed Internet access. Enhancements to the earliest DSL technology have led to Asymmetric DSL (ADSL), Very High Bit-rate Digital Subscriber Line (VDSL), and VDSL2, among others. The VDSL2 protocol is set forth in the International Telecommunications Union (ITU) standard G.993.2. VDSL2 employs Discrete Multitone (DMT) modulation, which is a form of orthogonal frequency division multiplexing (OFDM). In DMT modulation, the information-carrying frequency band associated with a communication is divided into multiple (e.g., up to several thousand) mutually orthogonal carrier frequencies, also referred to as tones or sub-carrier frequencies. As the sub-carrier frequencies into which a communication is divided are mutually orthogonal, there is no interference between them. However, there may be interference, i.e., crosstalk, between different communications occurring on neighboring wire pairs in a cable bundle. There are two types of crosstalk that can adversely affect a receiver: Far-End Crosstalk (FEXT) and Near-End Crosstalk (NEXT). FEXT is produced by neighboring wire pairs at a transmitter remote from the affected receiver. NEXT is produced by neighboring wire pairs at a transmitter local to the affected receiver.
Bit loading, a feature employed in VDSL transceivers, adaptively allocates the number of bits that are transmitted on each sub-carrier signal. The bit loading feature determines the number of bits to allocate to a sub-carrier signal in response to a measurement of signal-to-noise ratio (SNR) on the sub-carrier signal. Thus, the bit loading feature allocates more bits (and consequently, more bits per second per Hz) to a tone having a higher measured SNR than to a tone having a lower measured SNR. A common set of bit loading levels (i.e., numbers of bits that can be adaptively allocated) consists of integer bits between 1 and 15. A transceiver can adaptively select one of these bit loading levels based on the current measured SNR of a tone.
VDSL2 transceivers also employ a feature known as vectoring, which is a noise cancellation technique. Vectoring is used in VDSL2 specifically to cancel FEXT. Vectoring is described in ITU standard G.993.5, “Self-FEXT cancellation (vectoring) for use with VDSL2 transceivers” (2010).
VDSL and VDSL2 transceivers employ trellis coding, which is an error correcting technique. Trellis coding improves throughput under low SNR conditions. In trellis coding, the bits are divided between “payload bits” (also referred to as “user bits”) that encode the information or “payload” of the communication and trellis bits that carry the error-correcting information. For example, at a bit loading level of 15 bits, a trellis bit is allocated every two tones, effectively resulting in 14½ user bits per tone plus ½ trellis bit per tone. In an instance in which trellis coding results in improved throughput, such as in an instance in which SNR is low, the sacrifice of some user bits for trellis bits results in a net “coding gain.” Under the ITU VDSL standards, trellis coding is mandatory on all tones. Thus, conventionally, trellis coding is employed on all tones during all VDSL communications.